This year marks 125 years since the United Kingdom signed the Metre Convention. This is the treaty that provides the basis for international agreement on units of measurement. The Convention, produced by an International Commission, was signed on 20 May 1875 initially by the representatives of seventeen states: Argentina, Brazil, Peru, Venezuela, the USA, Ottoman Turkey, Russia and ten other countries in continental Europe. It provides for a permanent international Bureau of Weights and Measures and a General Conference, which now meets every four years.
The United Kingdom (of Britain and Ireland at that time) came late to the party and was the eighteenth signatory of the Convention in 1884.
The importance of the Convention is illustrated by the recent history of the yard and the pound.
Until 1893, the United States had attempted to maintain its standards of length and mass to be identical with those of the UK. However the fire at the Palace of Westminster in 1834, the resulting damage to Imperial fundamental standards, the lack of stability of the replacements, and the requirements for greater accuracy in measurement made the continuing alignment of the standards of the two countries increasingly difficult. Then in 1890, the US, as a signatory of the Metre Convention, received national prototype metric standards of length and mass. In 1893, it abandoned its national standards of the yard and the pound and adopted the following definitions:
One yard equals 3600/3937 of the metre
2.204 622 34 pounds avoirdupois equals one kilogram
The differences between US and Imperial measures of length and mass were a continuing problem for scientists and industry, and agreement was reached in 1959 between Australia, Canada, New Zealand, South Africa, the UK and the USA to establish uniformity in technical and scientific fields. These new definitions were agreed:
One yard equals 0.9144 metres
One pound avoirdupois equals 0.453 592 37 kilogram.
With the Weights and Measures Act 1963, the UK finally adopted these definitions for all purposes. Corresponding Imperial fundamental standards passed into history.
Of course, the Convention is not concerned only with length and mass, but has facilitated international agreement on standards for time, electricity, temperature, illumination and, for the atomic physicists, “substance”.
One question remains. When should we celebrate the benefits of a universal, simple, coherent system of measurement: on 20 May, the anniversary of the initial signing of the Convention, or on 10 October, a reminder of the link between the metric system and the number “10”?